dye sensitized solar cells (dsscs) and solar energy...

Dye Sensitized Solar Cells (DSSCs)

and solar energy conversion

The Solar Spectrum

2

Anatomy of the leaf

3

4

A spectrum of incident solar radiation at the Earth’s

surface featuring superimposed absorption spectra of

chlorophyll a (—), chlorophyll b (—), bacteriochlorophyll

a (—) and bacteriochlorophyll b (—).

[recorded in methanol/ethanol solution].

Light-Harvesting(energy transfer)

Reaction Center

(charge-separation via photoinduced e- transfer)

Light harvesting and electron transfer

Solar cell efficiencies (2013)

6

A different type of “solar system”

Light energy

input

Electron-transfer

Between subsystems

External circuit equilibrates

electrical energy

with surroundings

Current state-of-the-art

organic photovoltaics

8

Dye-sensitized solar cells (Grätzel cells)

• Dye-sensitized solar cells (DSSCs) are one of the most promising molecularphotovoltaics and have been attracting considerable attention since thepioneering study of Grätzel et al. because of the potential of low-costproduction.

• In order to improve the solar-to-electric power conversion efficiency (η),both metal complexes and metal-free organic dyes, as sensitizers ofnanocrystalline TiO2 electrode, have been extensively investigated anddeveloped in terms of optical absorption extension to the red or infraredregion through molecular design.

• The highest η for DSSCs has so far reached ~15 % obtained from an all solid-state perovskite sensitized TiO2 system.

ηηηη = maximum power output (Pmax )/ power input (Plight )

9

• A DSSC is comprised of a photoanode and cathode electrode, with an interstitial

redox mediator containing electrolyte, connected via an external circuit.

• The photoanode consists of an optically transparent electrode (OTE) surface

[typically fluorine-doped tin oxide (FTO) glass], with a mesoporous layer (ca. 10

mm thickness) of anatase TiO2 nanoparticles (ca. 10-20 nm diameter) boasting a

remarkable surface area (> 1000 cm-2).

• The dye sensitizer is adsorbed on the high surface area TiO2 by covalent

attachment allowing a large absorption cross section (light harvesting).

• This adsorption process utilizes appropriate functional groups on the dye molecule

for dye immobilization, carboxylic and phosphonic acid tethers being the most

commonly used.

• The anode (counter electrode) typically consists of platinized FTO glass.

Just like chlorophyll in natural photosynthesis the dye typically plays the role of

light harvester and electron transfer agent.

10

“Anatomy”

of a

DSSC

11

Light harvesting in DSSCs

12

State of the art in DSSCs

Snaith et al. Science 2012, 338, 643-647.

Graetzel et al. Nature 2013 499, 316-319.

CH3NH3PbI3 perovskite sensitized TiO2

Champion molecular based DSSC dyes

S SN

O

O

Se SO

OHN

OO

n-C6H12 n-C6H12

JSC = 17.94 mA.cm-2

VOC = 770 mV

FF = 0.730

= 10.3 %

C219 dye

N

N

N

NSCN

SCNRu

CO2H

CO2NaC106 dye

JSC = 19.78 mA.cm-2

VOC = 758 mV

FF = 0.779

= 12.10 %

S

S

ISC = short circuit current (mA)

A = area (cm-2)

JSC = JSC /A = short circuit current density (mA.cm-2)

VOC = open circuit voltage (mV)

FF = f ill factor

I0 = incident light f lux (mW.cm-2)

= power conversion eff iciency

Sn-C6H12

Sn-C6H12

JSC = 17.30 mA.cm-2

VOC = 965 mV

FF = 0.71

= 11.9 %

YD2-o-C8 dye

N

NN

N

N

C6H13

C6H13

O O

O O

CO2H

C8H17 C8H17

C8H17C8H17

Zn

Yu Q. e. atl ACS Nano. 2010, 4, 6032.

Zeng et. al Chemistry of Materials 2010, 22, 1915.

Yella et al. Science 2011, 334 (6056), 629-634.

Operation of Dye Sensitized Solar Cells

Graetzel M. Nature 2001, 414, 338.

• Charge separation and charge recombination are competitive!

E(V)vsNHE

(iv)

(vii)

Semiconductor band energies

16

• Band positions of several

semiconductors in contact with

aqueous electrolyte at pH 1.

• The lower edge of the

conduction band (red color)

and upper edge of the valence

band (green color) are

presented along with the band

gap in electron volts.

• The energy scale is indicated in

electron volts using either the

normal hydrogen electrode

(NHE) or the vacuum level as a

reference.

• On the right side the standard

potentials of several redox

couples are presented against

the standard hydrogen

electrode potential.

Semiconductor band bending

17

Schematic showing the electronic

energy levels at the interface

between an n-type semiconductor

and an electrolyte containing a

redox couple.

a: flat band potential

where no space-charge layer

exists in the semiconductor.


18





redox couple.

b: accumulation layer

where excess electrons have been

injected into the solid producing a

downward bending of the

conduction and valence band

towards the interface.


19





redox couple.

C: depletion layer

where electrons have moved from

the semiconductor to the

electrolyte, producing an upward

bending of the bands


20





redox couple.

d: inversion layer

where the electrons have been

depleted below their intrinsic

level, enhancing the upward band

bending and rendering the

semiconductor p-type at the

surface.

Scan rate 10mV/s. In each case: Fourth consecutive cycle depicted. Thickness: x and x , 4.5μm ; x, 4.4 μm

-50x10-6

-40

-30

-20

-10

0

10

20

30

CU

RR

EN

T D

EN

SIT

Y / A

cm

-2 µ

m-1

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2

ELECTRODE POTENTIAL vs. Ag/AgCl, 3M KCl

5.5 m LiTFSI in water ACID pH=2.6 ACID pH=3.6 NEUTRAL

Compare bare titanium dioxide currents

for 5.5m LiTFSI/water solutions of different pH.

Voltammogram shifts to positive directionwith increased acidity

Conduction band edge shift ,in conformity with theory

Blank TiO2 variable pH study

Fermi Level

http://www.acsu.buffalo.edu/~wie/applet/fermi/functionAndStates/functionAndStates.html

• Transparent (80%) in the visible

• Ideal electronic band energetics (wide bandgap semiconductor)

• High internal surface area (x700) / porosity (40-60%)

• Chemical and electrochemical stability

• “Right” surface chemistry for modification with dye sensitizers

• Mechanically rugged

• Good optoelectronic properties

• Commercially available

• “Proven” nanomaterial

The cathode material requires several combined properties and…

…undoped nanocrystalline (10 - 20nm) anatase TiO2 provides these properties.

Why TiO2 ?

• Screen-printed film

• 2-12 micron thick

• Uniform coating

• 80+% Transmission in the visible range

• Surface area ca. 85 m2/g

• Good adhesion to substrate

TiO2 films can be printed to achieve reliable device properties.

Preparation of TiO2 nanoparticle films

• TiO2 anatase

- nanoparticles size ~ 20 nm

- thin film thickness ~10 mm

8 hr.

Ti(i-PrO)4

dil. HNO3

TiO2 colloid

TiO2 Sol-gel

hydrolysis

peptidization

Autoclave 12 hrs. @ 400 0C

Film Deposition

Sintering 30 min. 450 0C

PEG addition

TiO2 mesoporous film

TiO2 anatase Raman

Rochford J.; Chu D.; Hagfeldt A.; Galoppini E. J. Am. Chem. Soc. , 2007, 129, 4655.25

blank & sensitized

TiO2 films with ZnTCPP

TiO2 & anchoring groups

26

Scanning electron micrograph of a

sintered mesoscopic TiO2 (anatase) film

supported on an FTO glass. The average

particle size is 20 nm.

Possible binding modes for a carboxylic acid anchoring group at TiO2 (M = Ti)

The mesoporous nanocrystalline effect

27

The incident-photon-to-current conversion

efficiency (IPCE) is plotted as a function of the

excitation wavelength.

a) Single-crystal anatase TiO2.

b) Nanocrystalline TiO2 anatase film.

FTO

Incident photon-to-current conversion efficiency

28

The (IPCE), sometimes referred to also as “external

quantum efficiency” (EQE), corresponds to the

number of electrons measured as photocurrent in

the external circuit divided by the monochromatic

photon flux that strikes the cell.

IPCEλλλλ = [(hc /e) JSC ]/[I0 λλλλ]

IPCEλλλλ = LHEλλλλ . φφφφinj . ηηηηcoll

LHEλ = light harvesting efficiency

φinj = quantum yield for charge injection

ηcoll = charge collection efficiency at FTO electrode

IPCE = monochromatic efficiency

Scattering Layer and TiCl4 Pre/Post-treatment

29

Deposition of a second inert layer of TiO2 is now standard practice to increase the

light harvesting efficiency.

Treatment with TiCl4 introduces a fresh layer of TiO2 across the FTO back

electrode and also increase conductivity throughout the mesoporous film

enhancing charge-collection kinetics – ultimately decreasing charge

recombination from FTO or TiO2 to the oxidized dye or mediator.

30

DSSC efficiency

ηηηη = Pmax / Plight

= (VOC . ISC . ff ) / (I0 . A) x 100

= (VOC . JSC . ff ) / I0 x 100

By increasing the resistive load on an

irradiated cell continuously from zero

(short circuit) to a very high value (open

circuit) one can determine the

maximum-power point, the point that

maximizes V×I; i.e. the load for which the

cell can deliver maximum electrical

power at that level of irradiation.

(The output power is zero in both the

short circuit and open circuit extremes)

VOC = open circuit voltage

ISC = short circuit current

A = area

JSC = short circuit current density = ISC / A

ff = fill factor = A1 / A2

I0 = incident light flux (e.g. AM 1.5)

A1

A2

Pmax

• Following absorption of incident light, the dye molecule undergoes rapid

transition (< fs) to an excited state electronic configuration.

• This metastable excited state (ττττ N719 = 12 ns) affords rapid charge injection (fs – ps)

from the singly occupied, lowest unoccupied molecular orbital (LUMO) into the

conduction band (CB) of the TiO2 semiconductor (ECB = -0.50 V vs. NHE) thus

creating a charge separated state, i.e. TiO2•−/dye•+ (charge injection).

• The ensuing fate of this exciton (electron/electron-hole pair) determines the

quantum yield for photocurrent generation and thus η for the DSC device.

• The excited state electron can diffuse to the back electrode FTO/TiO2 contact (ns –

ms) from where it is transported via an external circuit to the counter electrode. In

this scenario the ground state dye is regenerated by a redox mediator in the

electrolyte (typically I−/I3−, E0 = 0.35 V vs. NHE) and the circuit completed by

regeneration of the redox mediator at the counter electrode.

31

Kinetic competition in DSCs

(processes in kinetic competition have similar colors)

32

Nanorod & nanotube electrodes

base diameters ~ 40 nm

SEM images of ZnO nanotips films

Galoppini E.; Rochford J. et. al J. Phys. Chem. B, 2006, 110, 16159.

de Taccomi N. R. et. al J. Phys. Chem. C, 2009, 113, 2996.

Prof. K. Rajeshwar and Dr. N. de TaccomiThe University of Texas at Arlington

Prof. A. Hagfeldt & Dr. G. Boschloo, KTH StockholmProf. Y. Lu, Rutgers Piscataway.

Molecular engineering for DSSCs

• In general, the molecular structure of an organic dye for DSCs has been designed

and synthesized based on several basic concepts:

� energy matching of the highest occupied molecular orbital (HOMO) and the

lowest unoccupied molecular orbital (LUMO) of the dye with the energy levels

of I−/I3− redox potential and ECB of the TiO2 electrode, respectively.

� a donor-π-conjugation-linkage-acceptor (D-ππππ-A) structure required for a wide

range absorption extending to the near-infrared or infrared region.

� one or two anchoring groups such as carboxylic acid or phosphonic acid

groups are required for a strong adsorption onto the surface of TiO2.

� Steric/orientational design strategy for hindering charge recombination

processes.

34

• According to the reports of organic dyes used in DSSCs so far, the combination of a

triaryl amine (donor) with a cyanoacrylic acid (acceptor and anchor), brings about

good matching of the above-mentioned energy levels.

• Oligothiophenes have been successfully employed as the conjugation unit of the

dye molecule between the donor and acceptor groups, resulting in maximum

absorption bands shifting to the longer wavelength.

35

η = 10.3 %

Effect of Anchor Length on Charge Injection into TiO2

Prof. P. Piotrowiak at Rutgers University

Effect of Anchor Length on Open Circuit Voltage at TiO2

•The diode equation predicts a 59 mV increase in Voc for each order of magnitude decrease in the charge recombination rate constant for injected electrons with acceptors, ki[A]i, at room temperature provided that the electron injection flux into the semiconductor, Iinj, is constant.

•In efficient dye-sensitized solar cells with 1 sun irradiance (100 mW/cm2), the predominant acceptors are thought to be iodide oxidation products, I2 and/or I3

-

• Charge recombination rates decreased by a factor of 20 for 2 and 280 for 3 relative to 1 when studied in regenerative dye sensitized solar cells.

d = 7 Ǻ d = 18 Ǻ d = 24 Ǻ

Meyer G. J.; Galoppini E. et. al. J. Phys. Chem. B 2006, 110, 11044.

Fluorescence Emission Studies on Insulating ZrO2

550 600 650 700 750 800

ZrO2/Glass

p-ZnTCPP-[S]

m-ZnTCPP-[S]

m-ZnTCP2P-[S]

m-ZnTC(PEP)P-[S]

No

rmal

ised

Em

issi

on I

nte

nsi

ty

Wavelength (nm)

• ZrO2 behaves as an insulator precluding

electron injection from the lowest excited-state

of all porphyrins studied here.

(Ebg ~ 5 eV for ZrO2 ; Ebg ~3 eV for TiO2 and ZnO)

• H-type aggregation causes broadening

and convergence of emission bands.

• Aggregation can give rise to exciton diffussion

which may lower the charge injection efficiency.

• Emission bands of all meta-substituted porphyrins

remain resolved in contrast to p-ZnTCPP.

N N

NNZn

CO2- +NHEt3

CO2- +NHEt3

Et3HN+ -O2C

Et3HN+ -O2C

Photoelectrochemical Properties

400 500 600 700 8000

10

20

30

40

50

60

on TiO2/FTO

p-ZnTCPP-[S]

m-ZnTCPP-[S]

m-ZnTCP2P-[S]

m-ZnTC(PEP)P-[S]

IPC

E (

%)

Wavelength (nm)

cinjLHEIPCE ηφ ××=

1001240

)(% ××

×=

φλλ scI

IPCE

Photoelectrochemical properties of tetra(triethylammonium)carboxyporphyrin salts

PorphyrinIsc

(mA∙cm-2)

Voc

(V)ff

IPCE (%)

430 nm 570 nma 600nmb

p-ZnTCPP-[S] 0.39 0.44 0.54 18.50 1.44 (0.08) 0.86 (0.05)

m-ZnTCPP-[S] 3.33 0.51 0.41 58.60 29.40 (0.50) 16.30 (0.28)

m-ZnTCP2P-[S] 3.72 0.50 0.42 56.90 34.50 (0.61) 21.10 (0.37)

m-ZnTC(PEP)P-[S] 1.36 0.43 0.45 25.30 9.00 (0.36) 4.81 (0.19)

In parentheses are the a Q(1,0) and b Q(0,0) vs. Soret peak intensity ratios

• As the most widely used couple, I−/I3− has a favorable penetration ability into the porous

semiconductor film, fast dye regeneration and relatively slow recombination with injected

photoelectrons.

• Moreover, I−/I3− is the only redox couple which has been proven to have long term stability.

However, several disadvantages limit its industrial application:

� the corrosion of copper/silver lines, which are used to collect the electrons in large scale

modules making the production of long-term stable modules much more difficult

� the I3− and other possible polyiodides formed in the electrolyte absorb a considerable part of

the visible light, downgrading the efficiencies of DSSCs.

� The complex redox chemistry in the electrolyte causes great energy loss.

� The redox potential of I−/I3− limits the photovoltage.

To avoid these disadvantages, new kinds of redox couples have beendesigned and applied in DSSCs.

• The Voc can be significantly affected by the redox potential of the electrolyte.

� n is the number of photoelectrons in the TiO2

� NCB is the effective density of states of the TiO2 conduction band

� Eredox is the redox potential of the electrolyte redox couple

� q is the unit charge.

• Thus, at open-circuit conditions, the number of electrons in the TiO2, n, is determined by the

balance between electron injection and charge recombination.

• From the above eqn it can concluded that the Voc is determined by a selection of processes

� the irradiation level

� the properties of the semiconductor conduction band

� the injection efficiency of the dye

� the recombination loss rate of photoelectrons

� the redox potential of the electrolyte redox couple.

• Generally, n is considerably lower than NCB, and the maximum Voc is therefore determined by the

difference between the ECB of TiO2 and the Eredox of the electrolyte.

• Therefore, the redox potential of the redox system plays a key role in the Voc obtained in a DSSC.

• For the I−/I3− redox couple, the possibilities to increase the Voc are limited.

• Some modifications can be achieved by changing the concentrations and relative ratio of iodide and

iodine, but nevertheless the redox potential of the redox system is more or less fixed.

• Otherwise, in the conventional I−/I3− based

electrolyte, the redox mechanisms are

complicated. The regeneration is a multi

charge-transfer process whose exact

mechanism is still debated, but a two-step

process is generally accepted.

• In the first step of the dye-regeneration

process, the oxidized dye and iodide lead to

the formation of a diiodide radical (I2•−).

• The second step follows when the formed I2•−

disproportionates into I− and I3− . This leads

to a large loss in potential equal to the

difference between I− and I2•− redox

potentials (approximately 0.4 eV in

acetonitrile)

• CoII-complexes are commonly in the

high spin state, and CoIII-complexes

are in the low spin state.

• Therefore, CoII-complexes are more

reactive than CoIII-complexes.

• CoII in the electrolyte performs the

function of regenerating the

sensitizer in the photoanode

because of its higher reactivity.

• At the counter electrode, CoIII is

catalyzed to obtain an electron, and

converted to CoII.

• Co-based complex redox shuttles

have the advantage of a tunable

redox potential obtained through

modification of the ligands.

Co(III/II) mediators

• The efficiency obtained was initially

much lower than for DSSCs based on

the I−/I3− electrolyte.

• CoIII is more likely to experience

recombination with photoelectrons

from the TiO2 CB than I3−.

• Li+ as an electrolyte additive increases

the electron lifetime for a CoIII/II based

DSSCs due to a decrease in local conc.

of CoIII at the TiO2 surface.

• TBP (tert-butyl pyridine) facilitates

charge transfer from CoII to the dye

cation by reducing the reorganization

energy between CoII and CoIII.

• A thin Al2O3 blocking layer over the

mesoporous TiO2 surface has also

been used to minimize the charge

recombination


• A major disadvantage of CoIII/II basedDSSCs is how the photovoltaic

performance is constrained by mass-

transport limitations of the mediator.

• Elliot et al. have found that the

effective diffusion coefficient of

Co(dtb-bpy)33+ is about one order of

magnitude slower than that of I3−.

• This may be attributed to the greater

size and slower bulk diffusion of

Co(dtb-bpy)33+, greater viscosity of

cobalt complex solutions, and a

possible electrostatic surface

interaction of Co(dtb-bpy)33+/2+

within the TiO2 film.


• In 2009, Sun reported a new strategy of organic dye design by adding multiple alkoxyl

chains to the donor part of D–π–A dyes. The most efficient dye, D35, containing four

insulating butoxy chains efficiently prevent photoelectrons in TiO2 from recombining

with redox species in the electrolyte.

• In 2010, Hagfeldt and co-workers introduced the dye D35 to the Co-complex redox

system, an overall conversion efficiency of 6.7% with open-circuit potentials of more

than 0.9 V under 100 mW cm-2 AM1.5 G illumination using the combination of dye D35

and a Co(bpy)33+/2+ redox system.

• The key strategy was to reduce the recombination losses with blocking hydrophobic

groups on the sensitizing dyes, rather than, as in previous studies, on the Co

complexes.

• This approach allows the use of non-substituted Co complexes with higher diffusion

constants.

• The strategy thus successfully reduces both the recombination losses and, at least

partly, the mass-transport limitations.


• For best performance the TiO2 film porosity should be optimized.

• Organic dyes typically have higher extinction coefficients than the standard ruthenium

sensitizers, which makes it possible to use thinner TiO2 films in the DSSC devices.

• The use of a thinner TiO2 film is advantageous, since it decreases both the effects of

mass transport problems and the risk of recombination losses.

Steric control of TiO2 to Co(III) charge recombination

Current transients measured at an illumination intensity of 1000 W m-2 for DSSCs sensitized with D35

Current transients measured at an illumination intensityof 1000 W m-2 for [Co(bpy)3]

2+/3+ based DSSCs sensitized with D35 using a different thickness of TiO2.

Current density versus applied potential curves under 1000 W m-2

AM1.5G for DSSCs sensitized with D35. The electrolytes were 0.5 M Co(bpy)3(PF6)2, 0.1 M Co(bpy)3(PF6)3, 0.5 M TBP, and 0.1 M LiClO4 in acetonitrile or 0.6 M 1-butyl-3-methylimidazoliumiodide, 0.03 M I2, 0.5 M TBP, and 0.1 M guanidiniumthiocyanate in acetonitrile, respectively.

Record efficiency for a molecular DSSC sensitizerE(V)vsNHE

FTO

FTO


Device Fabrication by Graetzel Lab

Yella et al. Science 2011, 334 (6056), 629-634 (Taken from supp info).

• The photoanode used in this study consists of thin TiO2 electrodes comprising a 5

μm mesoporous TiO2 layer (particle size, 20 nm, pore size 32 nm) and a 5 μm

TiO2 scattering layer (particle size, 400 nm).

• Thickness and the porosity of the photoanode was found to be very crucial in the

cobalt based DSSCs. Porosity of the TiO2 paste was increased (from 23 nm pore

size to 32 nm pore size) and the thickness was reduced compared to I-/I3- based

devices to avoid the mass transport limitations in cobalt based devices.

• The working electrode was prepared by immersing the 10 μm (5 μm thick

transparent layer +5 μm thick scattering layer) TiO2 film into dye solution for 18 h.

• A thermally platinized FTO glass counter electrode and the working electrode were

then sealed with a 25 μm thick hot-melt film (Surlyn, by heating the system at

100°C. Devices were completed by filling the electrolyte by pre-drilled holes in the

counter electrodes and finally the holes were sealed with a Surlyn sheet and a thin

glass cover by heating. A black mask (6x6 mm2) was used in subsequent

photovoltaic studies.

Absorption spectra & CV of porphyrins YD2 and YD2-o-C8


Optimization of electrolyte composition


Performance at various light intensities (VOC depdenence)


J-V and IPCE DSSC characterization


APCE vs IPCE


Comparison of mediators for a YD2-o-C8 DSSC device


• Both Hagfeldt and Graetzel research groups have reported on the significant

efficiency enhancement of Z907 vs N719 when used with Co(III/II) mediators.

Ru sensitizers & Co(III/II) mediators

Liu et al. J. Phys. Chem. C 2011, 115, 18847-18855.

Feldt et al. J. Am. Chem. Soc. 2010, 132, 16714.

SnO2 – Ru– iodide – Pt based DSSCs ?

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

3.2

eV

TiO2

3.6

eV

SnO2ECB

ECB

EVB

EVB

EF

EF

I3 /IVOC VOC

TiO2 – Ru– iodide – Pt based DSSCs

SnO2 – Ru– iodide – Pt based DSSCs ?

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

3.2

eV

TiO2

3.6

eV

SnO2ECB

ECB

EVB

EVB

EF

EF

I3 /IVOC VOC

p-Type DSSCs (a reverse Graetzel cell)

E(V)vsNHE

charge

recombination

• Can we take a similar approach to p-type DSSCs ?

Voc = 350 mV in a NiO, I/I3 p-DSC system. A solar to electric conversion efficiency (Z) of 0.14% was achieved.

Tandem np-DSSCs

ECB

D0/D+

D*

hv

electron

transfer

e-

e-

e-

FTO TiO2 Dye (donor) Electrolyte Dye (acceptor) p-NiO FTO

EF

EVB

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

D0/D+

D*

hv

electron

transfer

e-

e-

e-

e-

Red Ox

Mediator e-

Voc''

dif f usion

charge

recombination

EF

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

e-

e-

Voc'

Voc = Voc' + Voc

''

e-

Current-density–voltage characteristics for dyes 1–3; green: 1; blue: 2; red: 3. in a p-type NiO device.c, Current–voltage characteristics of a tandem solar cell (black) as well as p-DSCs (red) and n-DSCs (green) under illumination (AM1.5, 1,000Wm 2, solid lines) and in the dark (dashed lines).Identical NiO and TiO2 films were used for the construction of the tandem, p- and n-DSC.

“By using suitable catalyzers, it should be

possible to transform the mixture of water

and carbon dioxide into oxygen and

methane, or to cause other endo-energetic

processes.”

International Conference of Applied Chemistry, New York,

September 11, 1912.

Solar Generation of Fuels

hν

H2O H2 + ½ O2

CO2 + 2H2O CH3OH + 1/2 O2

hν

Prof. Giacomo Ciamician (1857–1922)

“Godfather of photochemistry”

Prof. Ciamician surveys his collection of tubes and flasks

exposed to the sun on the balcony of his institute

At the University of Bologna from 1889 to

1922 G. Ciamician was inspired by the

ability of plants to make use of solar

energy, he was the first scientist to

investigate photochemical reactions in a

systematic way.

71

Heterogeneous Catalysts for

Water Oxidation, Proton reduction and CO2

reduction inspired by photosynthesis

photoanode

cathode

anode

cathode

Gray H., Nocera D., Nature 2008, 452.

H+ + e-→ H• E°= -2.20 V (1)

2H+ + 2e- → H2 E°= 0.00 V (2)

2OH• + 2H+ + 2e- → 2H2O E°= +2.72 V (3)

H2O2 + 2H+ + 2e- → 2H2O E °= +1.76 V (4)

O2 + 4H+ + 4e- → 2H2O E°= +1.23 V (5)

( E° @ pH 0 vs. NHE )

proton

reduction

+

water

oxidation

overall

water

splitting

Why study water oxidation ?

∆∆∆∆G≠ = overpotential

O2 + 2H2 ∆∆∆∆G0 = -nFEcell

E

Reaction coordinate

∆∆∆∆G≠ = overpotential

2H2O

2H2O → O2 + 4H+ + 4e- E0Ox = -1.23 V (1)

4H+ + 4e- → 2H2 E0Red = 0.00 V (2)

2H2O → 2H2 + O2 E cell = -1.23 V (3)

∆G0 = +237 kJ mol-1

Water Oxidation =

Photosynthesis

Light-Harvesting(energy transfer)

Reaction Center

(charge-separation via photoinduced e- transfer)

6CO2 + 6H2O + hv → C6H12O6 + 6O2

The oxygen evolving complex of PSII

H2O oxidation by oxygen evolving complex (OEC) via

proton-coupled e- transfer (dark reaction)

C. W. Hoganson and G. T. Babcock, Science 1997, 277 (5334), 1953-1956.

Water Oxidation and the Blue-Dimer

• Homogenous catalyst

• Electrochemically Oxidizes water in absence of

light

similar to Mn4CaO OEC in photosystem II

• Inefficient catalyst (only 25 turnovers)

• Mechanism has been extensively investigated by

Meyer, Hurst, Baik and others

• Proton-coupled oxidation of RuIII → RuIV → RuV

• Mechanism for O2 evolution still unknown

T.J. Meyer et al. JACS 1982, 104, 4030

RuIIIO

RuIII

OH2 OH2

(bpy)2 (bpy)2 RuIVO

RuIV

OH OH

(bpy)2 (bpy)2

RuVO

RuV

O O

(bpy)2 (bpy)2

RuIVO

RuIII

OH OH2

(bpy)2 (bpy)2

-H+, -e−−−− -H+, -e−−−−

-H+, -e−−−−

RuVO

RuIV

O OH

(bpy)2 (bpy)2

-H+, -e−−−−

?

2H2O

O2

WOC’s Inspired by Blue-Dimer

Llobet ’04

Thummel ‘06

Grätzel ‘87

• Loss of 4 e- and 4 H+ with

formation of O-O bond

• Low over-potential

• High turnover number

(stability)

• Steric requirements

•Tanaka dimer > 30,000 TN

“Forum on Making Oxygen” Inorg. Chem., 2008, 47, 1697-2232

Blue dimerT.J. Meyer ‘82

N NN

= terpyridine

Tanaka ‘03

N NN

= terpyridine

Q = 3,5-Cl2

Q = 3,6-tBu2

• Excellent electrocatalyst for water oxidation

• TN > 30,000 (1.7 V vs. Ag/AgCl @ pH 4)

• Unlike the Blue Dimer catalyst (RuIII/RuV) two Ru

atoms predominantly +2 during entire catalytic

cycle

• Active redox couples are SQ/Q

• Catalytic activity controlled by a delicate balance

of the charge distribution over the Q-Ru-OH

framework to make the O-O bond

• Steric hindrance is required to suppress the

deleterious Ru-O-Ru formation and allow the

desired Ru-O-O-Ru formation

• Substitution of bpy for the quinone ligands results

in negligible catalytic activity!!

The Tanaka Dimer

K. Tanaka et al., Inorg. Chem., 2001, 40, 329 & J. Am. Chem. Soc., 2003, 125, 6729.

Muckerman J. et al “Forum on making Oxygen”, Inorg. Chem. 2008, 47, 1787.

Tanaka monomer: Proton management !

Slope Description0 mV ET only 59 mV 1 e- 1H+ PCET30 mV 2 e- 1H+ PCET118 mV 1 e- 2H+ PCET

pKa aqua = 5.5 (calcd. 4.7)

pKa hydroxyl = 10.7 (calcd. 12.4)

(Square Wave Voltammetry)

pH0591.0)ln(*

| ⋅⋅−+=n

m

a

a

nF

RTEE

R

ORO

Tsai M.-K.; Rochford J.; Polyansky D. E.; Wada T.; Tanaka K.; Fujita E.; Muckerman J. Inorg. Chem., 2009, 48 (10), 4372.

• PCET occurs in PSII at OEC via adjacent Tyrosine residue (Tyz)

• PCET precludes build up of excess positive charge within the complex, i.e. at metal or ligand.

OH2

NN

NRu

O

O

II

2+

Mechanism of Tanaka Dimer for WOC (DFT)

Boyer J.; Rochford J.; Tsai M.-K.; Muckerman J.; Fujita E. Coord. Chem. Rev. 2010, 254, 309-330.

Tsai M.-K.; Rochford J.; Polyansky D. E.; Wada T.; Tanaka K.; Fujita E.; Muckerman J. Inorg. Chem., 2009, 48 (10), 4372.

Mononuclear water oxidation catalysts (WOC)

• Highly promising: less expensive, not as synthetically demanding

• Yet to be fully explored

J. J. Concepcion, J. W. Jurss, J. L. Templeton and T. J. Meyer. J. Amer. Chem. Soc. 2008, 130 (49), 16462-16463.

Natural

oxygen evolving complex

Artificial

oxygen evolving complex

S. Romain, L. Vigara and A. Llobet. Acc. Chem. Res. 2009, 42 (12), 1944-1953.

Requirements for CO2 reduction• One electron reduction of CO2 is unfavorable requiring an energy of -1.9 V.

• PCET and multi-electron transfer catalysts required to reduce large overpotentials

• Electricity as electron source (Electrochem)

• Photon as energy source (Photochem)

Comments Inorg. Chem. 1997, 19, 67

Coord. Chem. Rev. 1999, 185, 373

photoelectrochemistry

CO2 + 2H+ + 2e-

HCO2H

CO2 + e- CO2-

CO2 + 2H+ + 2e-

CO + H2O

CO2 + 4H+ + 4e- C + 2H2O

CO2 + 4H+ + 4e- H2CO + H2O

Reaction Eo (V)

-1.90

-0.61

-0.53

-0.20

-0.48

-0.38

-0.24

-1.55

-1.20

CO2 + 6H+ + 6e- CH3OH + H2O

CO2 + 8H++ 8e- CH4 + 2H2O

CO2-

+ e- CO22-

CO2 + 2e- CO22-

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)

Photocatalytic CO2 reduction by Re(I) carbonyls

• One of the most highly studied photocatalytic systems are the Re(I)

polypyridyl tricarbonyl systems first inspired by the Lehn catalyst.

ReCO

CO

CON

N

Cl

hv

TEOA

TEOA

CO

CO2

2 1 0 -1 -2

1 atm Ar

1 atm CO2

bkg Ar

bkg CO2

Potential (V) vs. NHE

Hawecker, J.; Lehn, J. M.; Ziessel, R., J. Chem. Soc. Chem. Comm. 1984, 328-330

• TN = 27

• TOF = 11 hr-1

• Φ = 0.14

Photocatalytic CO2 reduction by Re(I) carbonyls

• Ishitani has proposed a mechanism taking advantage of Re(bpy)(CO)3X complexes as both

photocatalyst and photosensitizer with one of the largest quantum yields reported to date at Φ = 0.59.

Takeda et al. J. Am. Chem. Soc. 2008, 2023.

Morris A.; Meyer G.; Fujita E. Acc. Chem. Res. 2009, 42, 1983-1994.

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